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United States Patent |
6,066,247
|
Sharma
|
May 23, 2000
|
Method for producing aluminum metal from aluminum trichloride
Abstract
A method is provided for the electrolytic production of aluminum metal from
aluminum trichloride starting material. A molten electrolytic solution of
mixed chloride-fluoride salts is formed in a reaction vessel into which
the aluminum trichloride feed material is added. The fluoride component of
the electrolyte reacts spontaneously with the aluminum trichloride,
producing aluminum fluoride and a chloride salt. The aluminum fluoride is
advantageously stored in a non-volatile state in solution in the
electrolyte. The aluminum fluoride is then electrolytically reduced to
yield aluminum metal and a fluoride salt.
Inventors:
|
Sharma; Ram A. (2951 Homewood Dr., Troy, MI 48098)
|
Appl. No.:
|
065636 |
Filed:
|
April 23, 1998 |
Current U.S. Class: |
205/372; 205/394 |
Intern'l Class: |
C25C 003/06 |
Field of Search: |
205/372,394
|
References Cited
U.S. Patent Documents
3103472 | Sep., 1963 | Slatin | 205/394.
|
3518172 | Jun., 1970 | Layne et al. | 205/394.
|
3725222 | Apr., 1973 | Russell et al. | 205/394.
|
Foreign Patent Documents |
891369 | Dec., 1959 | GB | 205/394.
|
Primary Examiner: Gorgos; Kathryn
Assistant Examiner: Parsons; Thomas H.
Attorney, Agent or Firm: Reising, Ethington, Barnes, Kisselle, Learman & McCulloch, P.C.
Claims
I claim:
1. A method of producing aluminum comprising the steps of:
preparing a molten salt electrolyte solution in a reaction vessel
comprising mixed metallic fluoride and chloride salts prepared from a
ternary mixture of MCl-MF-MAlF.sub.6 where M is an alkali metal element;
adding aluminum trichloride to the vessel and reacting the aluminum
trichloride with the electrolyte solution to produce aluminum fluoride
dissolved in the electrolyte solution; and
electrolytically reducing the aluminum fluoride to produce aluminum.
2. The method of claim 1 wherein the alkali metal element is selected from
the group consisting essentially of sodium, lithium, and potassium.
3. The method of claim 1 wherein the electrolyte solution comprises a
ternary eutectic composition of NaCl-NaF-Na.sub.3 AlF.sub.6.
4. The method of claim 1 wherein the electrolyte solution is selected from
the group consisting essentially of NaCl-NaF-Na.sub.3 AlF.sub.6,
LiCl-LiFLi.sub.3 AlF.sub.6, and KCl-KF-K.sub.3 AlF.sub.6.
5. The method of claim 1 wherein a sufficient amount of the metallic
fluoride salt is provided in the electrolyte solution to convert
substantially all of the aluminum trichloride added to the vessel to
aluminum fluoride.
6. A method of producing aluminum from aluminum trichloride starting
material comprising the steps of:
preparing a molten electrolyte salt solution in a reaction vessel
comprising a mixture of alkali metal chloride, alkali metal fluoride, and
alkali metal cryolite selected from the group consisting of
NaCl-NaF-Na.sub.3 AlF.sub.6, LiCl-LiF-Li.sub.3 AlF.sub.6, and
KCl-KF-K.sub.3 AlF.sub.6 ;
adding gaseous aluminum trichloride to the vessel and spontaneously
reacting the aluminum trichloride with the alkali metal fluoride
converting substantially all of the added aluminum trichloride to aluminum
fluoride and an alkali metal chloride, the aluminum fluoride being
dissolved in the electrolyte solution; and
electrolytically decomposing the alkali metal chloride and aluminum
fluoride to produce aluminum metal and an alkali metal fluoride.
7. The method of claim 6 wherein the electrolyte solution comprises
NaCl-NaF-Na.sub.3 AlF.sub.6 and the aluminum trichloride reacts with the
electrolyte according to the reaction:
AlCl.sub.3 +NaF.fwdarw.AlF.sub.3 +NaCl.
8. The method of claim 7 wherein the electrolytic reduction of AlCl.sub.3
to Al occurs according to the reaction:
3NaCl.fwdarw.3Na.sup.+ +1.5Cl.sup.-, and AlF.sub.3 +3Na.sup.+
.fwdarw.3NaF+Al.
9. The method of claim 6 wherein the electrolyte solution comprises
LiCl-LiF-Li.sub.3 AlF.sub.6 and the aluminum trichloride reacts with the
electrolyte according to the reaction:
AlCl.sub.3 +LiF.fwdarw.AlF.sub.3 +LiCl.
10. The method of claim 9 wherein the electrolytic reduction of AlCl.sub.3
to Al occurs according to the reaction:
3LiCl.fwdarw.3Li.sup.+ +1.5Cl.sup.-, and AlF.sub.3 +3Li.sup.+
.fwdarw.3LiF+Al.
11. The method of claim 6 wherein the electrolyte solution comprises
KCl-KF-K.sub.3 AlF.sub.6 and the aluminum trichloride reacts with the
electrolyte according to the reaction:
AlCl.sub.3 +KF.fwdarw.AlF.sub.3 +KCl.sub.3.
12. The method of claim 11 wherein the electrolytic reduction of AlCl.sub.3
to Al occurs according to the reaction:
3KF.fwdarw.3K.sup.+ +1.5Cl.sup.-, and AlF.sub.3 +3K.sup.+ .fwdarw.3KF+Al.
13. The method of claim 6 wherein there is sufficient amount of alkali
metal fluoride present in the electrolyte solution to convert
substantially all of the aluminum trichloride added to the vessel to
aluminum fluoride.
Description
This invention relates to a method of producing aluminum from aluminum
trichloride in a molten salt electrolyte solution.
BACKGROUND OF THE INVENTION
Fused salt electrolysis processes are used to produce metals. These
processes can be distinctively divided into two categories. In one
category a chloride is used as the feed material which is dissolved in a
molten chloride based electrolyte and then decomposed electrolytically to
produce metal on a suitable cathode and chlorine on a suitable anode. The
typical example of this type of process is the electrolytic production of
magnesium. In the second category, an oxide feed material is dissolved in
a molten fluoride-based electrolyte and then electrolytically decomposed
to produce metal on an appropriate cathode and oxygen on a carbon anode.
The carbon anode reacts with the oxygen to form carbon dioxide and is
consumed in the process. The typical example of this type of process is
the electrolytic production of aluminum by the Hall-Heroult process.
No process is known heretofore in which a chloride feed material is
dissolved in a fluoride-based electrolyte bath and then decomposed
electrolytically to produce metal on a suitable cathode and chlorine on a
graphite anode. Some advantages of such an approach would be: chlorides
are likely to have higher solubilities in the fluoride melts in comparison
with the limited solublities of oxides in these melts; the cell operating
temperatures should be lower; and in some cases, energy consumption may be
lower.
Aluminum has been traditionally produced by dissolving alumina in a
cryolite based electrolyte and decomposing it electrolytically at about
1000.degree. C. ever since the invention of the Hall-Heroult process in
1886. The cell operates at a comparatively high temperature and produces
fluoride compounds which pose serious health hazards.
A process using aluminum chloride as feed material and a chloride-fluoride
mixed electrolyte for dissolving and electrolyzing the chloride feed, may
eliminate both the problems mentioned above. In addition, there would be
about 25% savings in the energy consumption for producing aluminum, as
will be noted later on.
In the electrolytic production of aluminum according to the traditional
Hall-Heroult process, alumina (Al.sub.2 O.sub.3) is used as feed material.
It is prepared from bauxite. In the commonly used Bayer Process, bauxite
is finely ground in a ball-mill and stored in large bins. The ground
bauxite is next mixed with aqueous sodium hydroxide (NaOH) solution, of
specific gravity 1.45, in a vessel fitted with stirrers. After intimate
stirring, the mixture is pumped into steam-jacketed autoclaves and
digested for 2-8 hours under a pressure of 50 to 70 lbs per square inch,
at a temperature of 150 to 160.degree. C. The alumina of the bauxite
reacts with NaOH, forming sodium aluminate which goes into solution. After
the digestion is completed, the liquor from the autoclaves is forced into
large iron settling tanks and held 4-5 hours to allow settling of solid
impurities. This settled mass is called "red mud" and consists of
undissolved alumina, ferric oxide, titania, silica etc., from the bauxite.
The sodium aluminate liquor from the tanks is diluted from specific gravity
1.45 to 1.23, filtered and run into large precipitation tanks to
precipitate aluminum hydroxide. Sodium aluminate itself decomposes into
aluminum hydroxide and sodium hydroxide, but the decomposition is
accelerated by heating, stirring, and providing freshly prepared aluminum
hydroxide seeds in the precipitation tanks. The aluminum hydroxide
precipitation can take up to 60 hours. The aluminum hydroxide is separated
by filtration and dried in cylindrical rotary kilns at 1000 to
1100.degree. C. to prepare substantially pure anhydrous aluminum oxide for
use in the electrolytic cells.
The reduction cell or "pot" is a strong steel box, usually rectangular in
shape, and is lined with refractory insulation, which surrounds an inner
baked carbon lining. Thermal insulation is adjusted to provide sufficient
heat loss to freeze an electrolyte coating on the inner walls, to protect
them from the corrosive electrolyte attack. The bottom is not coated, and
remains bare for electrical contact with the molten aluminum cathode.
Steel collector bars are joined to the carbon container cathode, at the
bottom, to conduct electric current from the cell. Current enters the cell
through prebaked carbon anodes, or through a continuously self-baking
Soderberg anode.
The carbon lined pot serves as the cathode. The lining is made by two
methods. In one method, it is made by ramming a hot mixture of pulverized
metallurgical coke with tar and soft pitch binders into the steel shell,
using a suitable cast iron former to give the cavity the desired shape.
The entire pot is baked in a furnace at about 600 to 800.degree. C. In the
second method, preformed and prebaked carbon blocks are used to build up
the lining, the blocks being cemented together with a mixture of tar,
pitch, and ground coke. The cathode carries the current to the metallic
aluminum. Overheating and local stresses cause it to crack and degenerate.
Broken pieces of lining then float in the bath and cause partial short
circuits between the anodes and the metal. Breaks in the lining permit the
molten Al to attack the steel shell or collector box with the resultant
dissolution of the iron by the Al. When this happens, the cell is removed
and repaired. The carbon lining is porous and absorbs nearly its own
weight of the fused electrolyte. When the cells are repaired, the old
lining is broken out. The fused electrolyte is recovered by burning off
the carbonaceous materials in multi-hearth furnaces. Cells may operate as
long as 3 years, without repair or replacement.
The anodes are manufactured using calcined petroleum coke or hard pitch
coke instead of foundry coke. Prebaked anodes are produced by molding
petroleum coke and coal tar pitch binder into blocks (typically 70
cm.times.125 cm.times.50 cm) and baking to 1000-1200.degree. C. Steel
stubs seated in the anode support the anode in the electrolyte and conduct
electric current into the anodes. Electric resistivity of the anodes
ranges from 5-6 .OMEGA.-m anode current density 0.65-1.3 A/cm.sup.2.
The Soderberg type of anodes are continuously baked. The anode briquettes
are carried to the pots in portable hoppers, elevated and dumped into the
top of the anode shell. The temperature at the top of the pot is high
enough to melt the briquettes. The temperature in the lower part of the
anode shell is high enough to distill the volatile matter in the
briquettes and to bake the anode into a single solid block. The anode is
lowered into the pot, at a rate which compensates for the carbon used
during electrolysis. Its resistivity is about 30% higher than prebaked
anodes. Current density is lower, ranging from 0.65-0.9 A/cm.sup.2.
It is believed that alumina dissolves in cryolite at low concentrations by
the reaction,
Al.sub.2 O.sub.3 +4AlF.sub.6.sup.3- .fwdarw.3Al.sub.2 O F.sub.6.sup.2-
+6F.sup.- (1)
and at higher concentrations by the reaction,
2 Al.sub.2 O.sub.3 +2Al F.sub.6.sup.3- .fwdarw.3 Al.sub.2 O.sub.2
F.sub.4.sup.2- (2)
Ion transport measurements indicate that Na.sup.+ ions carry most of the
current. This is consistent with the lower decomposition potential of
sodium oxide (Na.sub.2 O) than that of aluminum oxide (Al.sub.2 O.sub.3)
in these ionic melts. Al is deposited on the cathode by the reaction,
12Na.sup.+ +4A1F.sub.6.sup.3- +12e.sup.- .fwdarw.12 (Na.sup.+ +F.sup.-)+4
Al+12 F.sup.- (3)
Oxyfluoride ions are discharged at the anode forming CO.sub.2 by the
reactions,
2Al.sub.2 O.sub.2 F.sub.4.sup.2- +C.fwdarw.CO.sub.2 +2Al.sub.2 O F.sub.4 +4
e.sup.- (4)
Al.sub.2 O F.sub.4 +Al.sub.2 O F.sub.6.sup.2- .fwdarw.Al.sub.2 O.sub.2
F.sub.4.sup.2- +2AlF.sub.3 (5)
Summation of equations (1) to (5) gives the overall reaction,
2Al.sub.2 O.sub.3 +3 C.fwdarw.4 Al+3 CO.sub.2 (6)
Faraday efficiency is reported to be 85-95%. Loss of efficiency is reported
to be caused by reduced species (Al, Na, or AlF) dissolving at the cathode
and being transported toward the anode where these species are reoxidized
by CO.sub.2, forming CO and metal oxide which can dissolve in electrolyte.
Aluminum is produced by carrying out electrolysis of alumina dissolved in
cryolite (3NaF.AlF.sub.3) based electrolyte at about 1000.degree. C. using
a carbon anode and a layer of molten aluminum as cathode. The electrolyte
consists of cryolite, 4-8 wt % CaF.sub.2, 5-13 wt % AlF.sub.3, 0-7 wt %
LiF, and 0-5 wt % MgF.sub.2. Joule heating from the flow of electric
current is more than enough to maintain the melt temperature. Molten
aluminum (sp. gr. 2.29 at 1000.degree. C.) is heavier than the electrolyte
(sp. gr. .about.2.095 at 1000.degree. C.). Therefore, a layer of aluminum
varying in thickness from a fraction of an inch to 4 to 5 inches serves as
the cathode at the bottom of the cell. A layer of 6-12 inches of molten
cryolite containing 2-5% alumina in solution serves as the electrolyte.
The lower end of the carbon anode is kept 2-4 inches above the upper
surface of the layer of molten aluminum. During the electrolysis, aluminum
forms at the aluminum cathode, and oxygen forms at the carbon anode.
Oxygen reacts with the carbon to form carbon dioxide.
Over the molten electrolyte is maintained a crust of frozen electrolyte
mixed with alumina which is added as the feed. As the concentration of
alumina in the electrolyte decreases by its consumption in the
electrolysis, alumina is added to the electrolyte by breaking the frozen
layer, on top of which has previously been distributed a layer of alumina.
Alumina has to be fine enough to be maintained in suspension long enough
to be dissolved by the agitation of the electrolyte. The time required for
200-mesh alumina to dissolve completely in fused electrolyte has been
found experimentally to vary between 1.5 to 9 minutes, depending on the
temperature, degree of saturation of electrolyte, and the character of
alumina. The theoretical emf for the dissociation of alumina in the
cryolite is 2.18 V at 1000.degree. C., but it becomes 2-3 times this value
in the cell operation.
Cathode current densities vary between 300-600 A/ft.sup.2 (0.32 to 0.65
A/cm.sup.2) and anode current densities, based on the face area, vary
between 5-7 A/in.sup.2 (0.77 to 1.1 A/cm ). Each cell takes a large
current at a low voltage; a number of cells are arranged in a line in
series. The line voltage may be 600-800 V. A line may contain 120 to 168
pots in series. Cell amperages are 34,000 to 130,000 amperes. Current
efficiency varies between 90 and 95%. The power requirements for aluminum
are 7.8 to 8.5 kWH dc or 8.3 to 9 kWH ac per pound. Carbon from 0.6 to 0.3
lb, alumina 2 lbs, and 0.03 to 0.05 lb of cryolite, are required for one
pound of aluminum production. At 100% efficiency, 1000 amp, produces each
24 hr day, 17.746 lb Al and 21.689 lb CO.sub.2, equivalent to 800 cu. ft
at 960.degree. C.
In 1973, Alcoa announced a new electrolytic process for producing aluminum,
(the Alcoa process) using an alkali and alkali earth chloride-based
electrolyte and aluminum trichloride (AlCl.sub.3 ) feed material. The
process consisted of 1) production of very pure alumina by the Bayer
process 2) chlorination of alumina for the production of aluminum
trichloride and 3) the electrolysis of the aluminum trichloride dissolved
in the electrolyte. Alumina with carbon was chlorinated in a reactor at
700-900.degree. C. The resultant aluminum chloride was purified and then
stored in the crystalline state in a tank. The cell consisted of a steel
enclosure, lined with a refractory material. The refractory material was
believed to be silicon oxynitride. The metal produced was collected in a
graphite compartment. The operating temperature was 700.+-.30.degree. C. A
typical composition of the electrolyte is reported to be 5 wt %
AlCl.sub.3, 53 wt % NaCl, 40 wt % LiCl, 0.5 wt % MgCl.sub.2, 0.5 wt % KCl
and 1 wt % CaCl.sub.2.
Several bipolar electrodes were stacked in the cell on top of each other at
an interpolar distance of approximately 1 cm. The anodically evolved
chlorine was used to sweep the aluminum from the cathodes. The pumping
effect of the chlorine bubbles also caused melt circulation and supply of
new electrolyte to the electrode compartments. The aluminum settled in the
bottom of the cell by falling counter-current to the chlorine gas. Alcoa
reported a current density of 0.8-2.3 A/cm and a typical single-cell
voltage of 2.7 V in comparison with the reversible decomposition potential
of 1.8 V. The ohmic voltage drop in the electrolyte was about 0.5 V. The
energy consumption was reported to be about 9 kWH/kg of Al including the
chlorination step energy consumption.
Though the Alcoa chloride electrolysis process was theoretically promising,
several difficult technical problems could not be solved satisfactorily.
One difficult problem was the production and handling of the very pure and
water-free aluminum trichloride. The process is no longer known to be in
operation.
SUMMARY OF THE INVENTION
The present invention uses the following criteria for the development of
the new process.
a) The new process should deviate minimally from the existing production
process in order to have a very high probability of successful
development;
b) The new process should use an electrolyte similar to that of the
existing process, and therefore can use the existing equipment, with minor
modifications;
c) The new process should use inexpensive, easily available,
non-hygroscopic raw materials as much as possible;
d) The new process should use as feed material a compound that will reduce
the energy consumption; and
e) The new process should reduce the number of operations involved to
minimize capital investment.
It is an object of the invention to provide a method for electrolytically
producing aluminum in which gaseous aluminum trichloride (AlCl.sub.3) is
used as the feed material in the place of alumina (Al.sub.2 O.sub.3 ) of
the existing process. The standard decomposition potential of AlCl.sub.3
is about 0.5 V lower than that of Al.sub.2 O.sub.3 . The lower
decomposition potential of AlCl.sub.3 plus the lower anode overvoltage is
believed to reduce the energy consumption by about 25% for aluminum
production in comparison to the energy consumption of the traditional
Hall-Heroult process.
It is essential, for the successful electrolytic decomposition of
AlCl.sub.3, to dissolve it in an electrolyte, store it, and then decompose
it, without decomposing any other component of the electrolyte. This is
done according to the invention by using electrolyte melts comprising
fluorides and a chloride. A suitable fluoride is used in the electrolyte
to chemically react with the gaseous AlCl.sub.3 feed material, for
dissolving and storing it in a non-volatile state. The fluoride ions also
cleanse the aluminum produced to the maximum possible extent. A chloride
is used in the electrolyte to produce a metal which will generate Al by
chemically reacting with aluminum fluoride (AlF.sub.3).
Thus, according to the invention, there is provided a method for producing
aluminum metal in which aluminum trichloride (preferably gaseous) is added
to a molten mixture of chloride-fluoride based electrolyte salt solution
whereupon the aluminum trichloride reacts spontaneously with the fluoride
component producing a chloride salt and aluminum fluoride which is
dissolved in the electrolyte. The aluminum fluoride is electrolytically
reduced to produce aluminum and a fluoride salt.
According to a further feature of the invention, the molten electrolyte
salt solution comprises a mixture of alkali metal chloride, alkali metal
fluoride, and alkali metal cryolite, with the alkali metals comprising
preferably sodium, lithium and potassium. The ternary electrolyte system
is preferably a eutectic composition with the electrolyte maintained at an
operating temperature above the eutectic temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects and advantages of the invention will be understood in view of
the following detail description and the figures, in which:
FIG. 1 is a graph showing the decomposition potential of various salts
versus temperature;
FIG. 2 is a graph showing the free energy change of certain reactions
versus temperature;
FIG. 3 is a graph showing the decomposition potential of various compounds
as a function of temperature;
FIG. 4 is a graph showing the free energy change of various reactions as a
function of temperature;
FIG. 5 is a graph showing the decomposition potential of various compounds
as a function of temperature; and
FIG. 6 is a graph showing the decomposition potential of various potassium
compounds as a function of temperature in relation to the decomposition of
aluminum fluoride.
DETAILED DESCRIPTION
A method for producing aluminum from aluminum chloride starting material
involves preparing a mixture of chloride-fluoride molten electrolyte salt
solution in a reaction vessel, and adding to the vessel the aluminum
chloride feed material which reacts spontaneously with the fluoride
component of the electrolyte, producing chloride salt and aluminum
fluoride dissolved in the electrolyte solution. The aluminum fluoride is
then electrolytically decomposed to produce aluminum metal and a fluoride
salt.
The combination of the aluminum chloride feed material and the electrolyte
solution is unique in that the electrolyte is one having a fluoride salt
constituent which, when the aluminum trichloride is added to the
electrolyte, readily and spontaneously reacts with the aluminum
trichloride converting it into aluminum fluoride, thus dissolving and
storing the aluminum trichloride in a non-volatile state in the
electrolyte solution. Converting the initial feed material to aluminum
fluoride eliminates aluminum chloride as a component of the electrolyte
solution, overcoming the problems associated with the high volatility and
low ionic conductivity associated with the electrolytic reduction of
aluminum trichloride, according to traditional techniques. The aluminum
fluoride is advantageously stored in a non-volatile state in the
electrolyte and its ionic conductivity is greater than that of aluminum
trichloride, increasing the efficiency and reducing the energy consumption
in the production of aluminum.
A preferred electrolytic solution according to the invention is one that
includes a mixed fluoride-chloride salt, preferably alkali metal fluoride
and chloride salts. Still further it is preferred that the electrolyte
solution comprise a ternary eutectic composition of such alkali metal
chloride and fluoride salts along with an alkali metal cryolite compound,
the alkali metals selected preferably from the group consisting
essentially of sodium, lithium and potassium.
According to a particular embodiment, the ternary sodium chloride
(NaCl)-sodium fluoride (NaF) -aluminum fluoride (AlF.sub.3) phase diagram
indicates a ternary eutectic of 51.85 wt % NaCl, 15.6 wt % NaF and 32.55
wt % of Na.sub.3 AlF.sub.6, at 674.degree. C. This eutectic melt is
believed to be suitable for use as an electrolyte in the cells operating
at about 750.degree. C.
The standard free energy change of the reaction of AlCl.sub.3 with NaF has
a sufficient negative standard free energy change that the reaction is
spontaneous. Thus, on addition of AlCl.sub.3 to the above ternary melt
electrolyte, the following reaction,
AlCl.sub.3 +3NaF.fwdarw.AlF.sub.3 +3NaCl (7)
should occur spontaneously, forming aluminum fluoride and sodium chloride.
The melt now consists of sodium chloride (NaCl), sodium fluoride (NaF) and
aluminum fluoride (AlFl.sub.3 ) and does not have the AlCl.sub.3 as a
component, with its volatility and low ionic conductivity problems. On
applying a suitable potential to carry out the electrolysis, NaCl should
decompose electrolytically by the reaction,
3NaCl.fwdarw.3Na+1.5 Cl.sub.2 (8)
as is indicated by its decomposition potential, which is lower than that of
any other component of the electrolyte, as shown in FIG. 1.
The electrolytically produced sodium (Na), by reaction (8), should react
with aluminum fluoride (AlF.sub.3) by the reaction,
AlF.sub.3 +3Na.fwdarw.3NaF+Al (9)
forming sodium fluoride (NaF) and Al. The spontaneity of this reaction is
indicated by its negative standard free energy change, shown in FIG. 2.
The net result of reactions (7) to (9) is the reaction,
AlCl.sub.3 .fwdarw.Al+1.5 Cl.sub.2 (10)
whose standard decomposition potential as a function of temperature is
given in FIG. 3. During electrolysis, AlCl.sub.3 should decompose without
decomposing any of the other components of the electrolyte melt, as
indicated by its lowest standard decomposition potential (FIGS. 1 and 3).
The binary lithium chloride-lithium fluoride (LiCl-LiF), sodium
chloride-sodium fluoride (NaCl-NaF) and potassium chloride-potassium
fluoride (KCl-KF) systems are very similar, i.e. each system forms a
simple eutectic. The binary lithium fluoride-aluminum fluoride
(LiF-AlF.sub.3), sodium fluoride-aluminum fluoride (NaF-AlF.sub.3) and
potassium fluoride-aluminum fluoride (KF-AlF.sub.3 ) systems are also
similar, i.e. each system forms a cryolite compound and a eutectic between
the alkali fluoride and the compound. The binary lithium chloride-lithium
cryolite (LiCl-Li.sub.3 AlF.sub.6), sodium chloride-sodium cryolite
(NaCl-Na.sub.3 AlF.sub.6), and potassium chloride-potassium cryolite
(KCl-K.sub.3 AlF.sub.6), are similar too, i.e., each system forms a simple
eutectic.
Therefore, ternary LiCl-LiF-Li.sub.3 AlF.sub.6, NaCl-NaF-Na.sub.3
AlF.sub.6, KCl-KF-K.sub.3 AlF.sub.6 systems should also be very similar.
The ternary Li.sub.3 AlF.sub.6 -(LiF).sub.3 -(LiCl).sub.3 phase diagram
indicates a large region of melts having melting points below 700.degree.
C.
Using the above phase diagrams, appropriate melts of LiCl-LiF- Li.sub.3
AlF.sub.6 and KCl-KF-K.sub.3 AlF.sub.6 can be found as suitable
electrolytes, for producing aluminum at temperatures between
675-800.degree. C. In the case of low melting electrolytes consisting of
LiCl-LiF- Li.sub.3 AlF.sub.6 and KCl-LiF- Li.sub.3 AlF.sub.6 melts, the
reactions analogous to those described for the NaCl-NaF - Na.sub.3
AlF.sub.6 electrolytes are as follows. On addition of gaseous AlCl.sub.3
in the cell, the reaction,
AlCl.sub.3 +3LiF.fwdarw.AlF.sub.3 +3LiCl (11)
should occur, spontaneously, as indicated by its negative free energy
change (FIG. 4). On applying a suitable potential to carry out the
electrolysis, LiCl will decompose electrolytically by the reaction,
3LiCl.fwdarw.3Li+1.5 Cl.sub.2 (12)
This is indicated by its standard decomposition potential, which is lower
than that of any other component of the electrolyte in FIG. 5. The
electrolytically produced lithium should react with AlF.sub.3 in the
electrolyte melt producing aluminum by the reaction,
AlF.sub.3 +3Li.fwdarw.3LiF+Al (13)
The possibility of this reaction is again indicated by its negative
standard free energy change shown in FIG. 2. The net result of reactions
(11) to (13) is again the electrolytic decomposition of AlCl.sub.3 by
reaction (10), as described earlier.
In the case of the electrolyte melts of higher melting temperatures, i.e.
KCl-AlF.sub.3 -K.sub.3 AlF.sub.6, the reactions are as follows. Upon
addition of AlCl.sub.3 feed material to the electrolyte in the cell, the
reaction,
AlCl.sub.3 +3KF.fwdarw.AlF.sub.3 +3KCl (14)
should occur, spontaneously, as is indicated by its negative standard free
energy change (FIG. 4). On applying a suitable potential to carry out the
electrolysis, KCl will decompose electrolytically by the reaction,
3KCl.fwdarw.3K+1.5 Cl.sub.2 (15)
This is indicated by its standard decomposition potential in FIG. 6, which
is lower than that of any other component of the electrolyte. The
electrolytically produced potassium (K) should react with AlF.sub.3 in the
electrolyte melt, producing aluminum by the reaction,
AlFl.sub.3 +3K.fwdarw.3KF+Al (16)
The possibility of this reaction is again indicated by its negative
standard free energy change, as shown in FIG. 2. The net result of
reactions (14) to (16) is again the electrolytic decomposition of
AlCl.sub.3 by reaction (10) as described before.
Cryolite (Na.sub.3 AlF.sub.6) is reported to dissociate as follows:
Na.sub.3 AlF.sub.6 .fwdarw.3Na.sup.+ +AlF.sub.6.sup.3- (17)
and
AlF.sub.6.sup.3- .rarw..fwdarw.AlF.sub.4.sup.- +2F (18)
Sodium chloride dissociates as,
NaCl.fwdarw.Na.sup.+ +Cl.sup.- (19)
and sodium fluoride dissociates as,
NaF.fwdarw.Na.sup.+ +F.sup.- (20)
Therefore the melts composed of NaCl, NaF, and Na.sub.3 AlF.sub.6 should
consist of Na.sup.+, Cl.sup.-, F.sup.-, AlF.sub.4.sup.- and
AlF.sub.6.sup.3- ions. Structurally, these melts are simpler than those of
cryolites containing alumina.
Their ionic conductivity should be far better than cryolite-alumina melts
because of the presence of plentiful Na.sup.+, Cl.sup.-, F.sup.- ions. As
electrolytes they should sustain far higher current densities than those
of cryolite-alumina melts that are now used in aluminum production.
As pointed out earlier, AlCl.sub.3 on addition to the electrolyte should
react with NaF forming AlF.sub.3 and NaCl by reaction (7). As in the case
of cryolite melts, AlF.sub.3 should further react with F.sup.- by the
reaction forming possibly F.sub.6.sup.3-
AlF.sub.3 +3F.sup.- .fwdarw.AlF.sub.6.sup.3- (21)
Ion transport measurements indicate that Na.sup.+ ions carry most of the
current in the electrolysis of cryolite-alumina electrolytes used for
producing aluminum at the present. Al metal is deposited on the cathode,
by the reaction,
3 Na.sup.+ +AlF.sub.6.sup.3- +e.sup.- .fwdarw.3 (Na.sup.+
+F.sup.-)+Al+3F.sup.- (22)
In the NaCl-NaF-Na.sub.3 AlF.sub.6 melts, and similar melts of the present
invention, a similar reaction should occur at the cathode, involving the
reduction of AlF.sub.4.sup.- - instead of AlF.sub.6.sup.3-, as follows:
3 Na.sup.+ AlF.sub.4.sup.- +3e.sup.- .fwdarw.3
(Na.sup.+F.sup.-)+Al+F.sup.-(23)
At the anode, chlorine may be generated by the reaction,
3NaCl+AlF.sub.6.sup.3- +F.sup.- .fwdarw.3Na.sup.+ +AlF.sub.4.sup.-
+3F.sup.- +1.5 Cl.sub.2 +3e.sup.- (24)
Summation of AlCl.sub.3 dissolution reaction (7) and (21), the cathode
reaction (22), and anode reaction (23) gives the overall reaction (10).
Similar arguments can be advanced for other electrolytes such as
LiCl-LiF-Li.sub.3 AlF.sub.6 melt, KCl-KF-K.sub.3 AlF.sub.6 melt, and other
similar melts.
Accordingly, a 25% energy saving in the aluminum production may be
recognized by using aluminum trichloride as feed material. Currently,
about one third of the capital investment and the plant are used for
producing Soderberg anodes. Aluminum produced according to the invention
eliminates the need for costly Soderberg anodes.
Millions of dollars are being spent in research and development to procure
non-consumable anodes without much success. The process of the present
invention employs a non-consumable graphite anode when using pure aluminum
trichloride, or slightly consumable anode when using impure aluminum
trichloride.
Finally, the process of the present invention requires fewer operations
compared to existing aluminum production processes, thus simplifying the
production of aluminum.
The disclosed embodiment is representative of a presently preferred form of
the invention, but is intended to be illustrative rather than definitive
thereof. The invention is defined in the claims.
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